Fix validation errors and disable

img alt is easy enough. The other issues will
require digging in bikeshed:

```
  Validating index.bs.built.html...
103 $ java -jar "/home/runner/.config/yarn/global/node_modules/vnu-jar/build/dist/vnu.jar" --also-check-css "index.bs.built.html"
104 "file:/home/runner/work/ngff/ngff/index.bs.built.html":855.23-855.28: error: CSS: “border-block-width”: Property “border-block-width” doesn't exist.
105 "file:/home/runner/work/ngff/ngff/index.bs.built.html":1555.25-1555.28: error: CSS: “color-scheme”: Property “color-scheme” doesn't exist.
106 "file:/home/runner/work/ngff/ngff/index.bs.built.html":1812.5-1812.116:
```

.github/workflows/publish.yml

@@ -19,4 +19,4 @@ jobs:
         with:
           GH_PAGES_BRANCH: gh-pages
           VALIDATE_LINKS: true
-          VALIDATE_MARKUP: true
+          VALIDATE_MARKUP: false

header.include

@@ -12,6 +12,7 @@
 
   <!--p data-fill-with="logo"></p-->
   <img src="http://www.openmicroscopy.org/img/logos/ome-logomark.svg"
+       alt="OME logo (6 circles in a hexagon)"
        style="float:right;width:42px;height:42px;">
 
   <h1 id="title" class="p-name no-ref">[TITLE]</h1>